The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
An airplane is driven by several turbojet engines each housed in a nacelle also harboring an assembly of ancillary actuation devices related to its operation and providing various functions when the turbojet engine is operating or is at a standstill, such as for example a thrust reversal system.
A nacelle generally has a tubular structure comprising an air intake upstream from the turbojet engine, a middle section intended to surround a fan of the turbojet engine, a downstream section intended to surround the combustion chamber of the turbojet engine and if necessary harboring the thrust reversal means, and generally ends with an ejection nozzle, the outlet of which is located downstream from the turbojet engine.
Modern nacelles are intended to harbor a double flow turbojet engine capable of regenerating, via the blades of the rotating fan, a hot air flow (also called primary flow) stemming from the combustion chamber of the turbojet engine, and a cold air flow (secondary flow) which circulates outside the turbojet engine through a ring-shaped passage, also called vein, formed between a fairing of the turbine engine and an internal wall of the nacelle. Both air flows are ejected from the turbojet engine through the rear of the nacelle.
The role of a thrust reverser is, during the landing of an airplane, of improving the braking capacity of the latter by redirecting forwards at least one portion of the thrust generated by the turbojet engine. In this phase, the reverser blocks the vein of the cold flow and directs the latter towards the front of the nacelle, consequently generating a counter-thrust which will be added to the braking of the wheels of the airplane.
The means applied for achieving this reorientation of the cold flow vary depending on the type of reverser. However, in all the cases, the structure of a reverser comprises moveable cowls which may be displaced between a deployed position in which they open in the nacelle a passage for the deflected flow on the one hand, and a retracted position on the other hand in which they close this passage. These cowls may fulfill a deflection function (reverser with pivoting doors) or simply for actuating other deflection means.
In the case of a reverser with grids, also known as a cascade reverser, the reorientation of the air flow is carried out by deflection grids, the cowl only having a simple sliding function aiming at uncovering or covering these grids. Additional blocking doors also called foil flaps, activated by the sliding of the cowling, also allow closing of the vein downstream from the grid so as to optimize the reorientation of the cold flow.
In order to support the moveable reversion cowls and to bind the downstream section to the remainder of the nacelle and notably to the middle section via the fan case, the downstream section comprises fixed elements and notably longitudinal beams bound upstream to a substantially ring shaped assembly called a front frame, formed with one or several portions between said longitudinal beams, and intended to be attached to the periphery of the downstream edge of the case of the fan of the engines.
In the case of a grid reverser with translationally moveable cowls, the supporting fixed element will typically comprise two upper longitudinal beams said to be at 12 o'clock, positioned on either side of a nacelle attachment pylon or of a pylon type interface, and two lower longitudinal beams said to be at 6 o'clock.
These beams are also used as a support for translation guiding rails of the moveable cowls.
The front frame is then formed with two substantially hemicylindrical half frames binding the upper and lower beams together on either side of a longitudinal axis of the nacelle.
In the case of a thrust reverser of the cascade or grid type, this front frame is also used for supporting the assembly of deflection grids positioned between said beams.
In the thrust reversal position, the deflected air flows through the deflection grids from the circulation veins between the front frame and an upstream edge of the moveable cowl having moved backwards.
Thus, in order to maximize the thrust reversal performances, the front frame should have an aerodynamic profile promoting laminar flow of the deflected stream and generate as few as possible aerodynamic accidents on the flow path of the deflected air. This is why, it has a curved profile, at least in the proximity of the circulation vein of the airflow its deflection initiation. This curve portion is called a deflection edge.
Certain line accidents in a forward thrust mode cannot be suppressed and therefore have to be minimized.
A first line accident exists because of the mounting of the front frame on the fan case. In addition to difficulties in perfectly aligning the front frame and the case at the interior of the vein, it should be noted that in order to avoid scooping of the air by the front frame, an upstream edge of the interface between the front frame and the case should be radiant outwards.
In fact, the result thereof is necessarily the presence of a cavity, forming an aerodynamic accident in the vein.
In the same way, there also necessarily exists functional play between this front frame and the blocking foil flaps which, in the closing position have to come and restore at best the inner aerodynamic continuity of the vein. In order to avoid any scooping, a downstream edge of the blocking foil flaps is also curved towards the outside of the nacelle, thereby creating a second cavity.
However it should be noted that the foil flaps are moveable around a downstream pivot axis and that the interface for mounting the foil flaps is not sealed. Therefore there will necessarily exist minimum scooping through the cavities for housing the blocking foil flaps.
Thus, in non-reversed flow, said to be a direct flow, the air flow encounters at least two line accidents, respectively at an interface between the front frame and the fan case, and at an outcrop between the front frame and the blocking flaps.
In the thrust reversal position, the deflected stream remains perturbed by the first cavity which, by perturbing the flow of the air stream, causes a slight delay in the deflection of the airflow which no longer perfectly adheres to the deflection edge of the front frame.
It is therefore understood that both of these performances in forward thrust mode and in reversed thrust mode may be impacted by these aerodynamic accidents.
A first solution for simply addressing this problem is to raise a lower portion of the front frame so as to generate a pocket for receiving therein the upstream portion of the blocking foil flaps.
This solution is notably described in document U.S. Pat. No. 4,185,798.
A limitation of such a solution however lies in the fact that in the thrust reversal mode, the air stream tends to pursue its direct path and not re adhere to the deflection edge, which substantially reduces the efficiency of the thrust reversal device.